Ore no Oniku-chan

Kou Ryuugasaki is as fat as a meatball, and has a huge crush on his high school senpai, Chiaki Hiiragi, who calls him "Oniku-chan." On Chiaki’s graduation day, Kou finally works up the courage to... not quite ask him out, but is horribly rejected!

After working to lose all of his weight, Kou is now new and improved, but when he finally runs into Chiaki, he discovers that Chiaki is and always has been a chubby chaser?! Now they're finally dating, but Chiaki is asking him to fatten up again, and Chiaki’s ex-boyfriend has also entered the mix...!

(Source: Ichijinsha, translated)